1. Field of the Invention
The present invention relates to an injection rate controller for a fuel injection pump, which is adapted to lower the noise level under a specific condition of engine drive such as at engine idle, for example.
2. Description of the Prior Art
The fuel injection rate of a fuel injection pump mounted on a diesel engine, i.e. the injection quantity per unit crank angle, is substantially determined by the diameter of a plunger and the profile of a face cam in case where the pump is of a distributor type. Since the injection rate cannot be controlled even though the operation conditions are optionally set, when fuel supplied under pressure from the fuel injection pump is injected through an injection nozzle into a combustion chamber and combusted therewithin, the initial injection rate becomes high and the combustion speed becomes temporarily high to thereby heighten the heat generation rate and increase the noise level. The tendency to these adverse phenomena is markedly developed under idle operation such as at the time of warm-up. Particularly, during the wintertime, the engine noise reaches a high level. Therefore, it has been desired to reduce or eliminate the drawbacks suffered by the conventional engines.
There has heretofore been proposed an injection rate controller capable of varying the fuel injection rate by causing part of the fuel sucked in the interior of a plunger to escape into a pump house in response to the engine load conditions. For example, U.S. Pat. No. 4,413,600 discloses a distributor type fuel injection pump which comprises a plunger having first and second cut-off ports opening in the peripheral surface thereof and communicating with a high pressure chamber, a control sleeve adapted to open and close the cut-off ports and provided in the peripheral surface thereof with a plurality of spill ports smaller in number than the cylinders of an engine, the spill ports being normally stopped up and, at engine idle, being successively communicated with the second cutoff port to cause part of the fuel pressurized within the high pressure chamber to escape into a pump house, whereby low injection rate at engine idle can be secured. This prior art, however, necessitates formation of the first and second cut-off ports in the plunger and a plurality of spill ports in the control sleeve and, therefore, entails a problem that complicated and troublesome processing should be effected relative to the plunger and the control sleeve.